bodon My scope has an f ratio of 4.93 with reducer. Just a tad confused as the picture on the ZWO website says 'suitable for telescopes up to f 5.4.
Since you are using a reducer, it will depend on the optical design of the reducer.
For a simple lens (e.g., telescope objective with no reducer), this is what the light cones look like.
The distance between the aperture and the sensor is the focal length, and when divided by the size of the aperture, is the f-number. In the above diagram, it is approximately f/2.
The red and green dashed lines show the light cones from two skewed rays that reach the corners of the sensor.
Any iris that is placed between the lens and the sensor will need a hole that is at least beyond the outer red and outer green dashed lines to avoid any aperture vignetting.
So, it depends on the f-number and where you place the iris. The f-number determines how "fat" each cone is.
However, with a reducer, the f-number does not always determine the angle of the cone, nor is the aperture the aperture of your objective. You can see this by looking at the simple compound optics shown at the bottom of this page; notice that the effective aperture is no longer the aperture of the original objective (most of the glass on the right handside lens is not used):
https://www.schoolphysics.co.uk/age16-19/Optics/Refraction/text/Lenses_focal_length_measurement/index.html
Since precision glass is expensive, you can expect the aperture to be represented by the opening of the reducer -- it won't be much larger. Measure that, and draw the same diagram as I had drawn, but using the back focal distance of your reducer as the distance between the "aperture" and the sensor. This will tell you how much the opening (drawn as "iris" in my diagram) is need for the given position of the opening from the sensor.
Keep in mind too that the "iris" for a 36mm ZWO EFW is not 36mm. It is 34mm. A little less if you want to be pedantic, because of the thickness of the carousel plate.
If the last glass element of your reducer is less than 34 mm, you should not have any vignetting. I.e., if there is any vignetting, it has already be caused by the reducer itself. Furthermore, the image circle of a telescope with a reducer is usually less than the image circle of the telescope without the reducer. Good reducers will specify their image circles, and it might be even less than 34 mm, too.
As usual, when you mix and match systems, there is never a general answer.
Chen
